This invention relates to a method and a device for suppression of leakage of magnetic flux in a cathode ray tube display apparatus, which is adapted to suppress leakage magnetic flux generated from the deflection yoke.
There are many display apparatuses of the type utilizing a magnetic field generated from the deflection coil, etc. as the method of deflecting an electron beam. For this reason, in the display apparatus of this kind, a magnetic flux from the deflection coil, etc. leaks out from the surrounding of the cathode ray tube. Such an unnecessary electromagnetic ray may exert bad or adverse influence on the operation of electronic equipment in the vicinity thereof.
Recently, studies on the influence exerted on the human being and animals of the magnetic field have been conducted. For example, a magnetic field from the cathode ray tube is considered to be also harmful. In view of this, the research institution of SSI (Statens Stralskydds Institut) in Sweden has announced that in regard to the super low frequency magnetic field, with the point within 15 cm from the center of the display surface of the cathode ray tube being as a center, as recommended values of the change rate, i.e., time-derivative (dB/dt) of a magnetic flux density at 80 positions positions on the spherical body having a radius of 65 cm and the strength of a magnetic flux density in the frequency band of 10 to 300 KHz, the change rate of the maximum magnetic field is set to a value lower than 25 mT (mili tesla), and the strength of the maximum magnetic flux density is set to a value lower than 50 nT (nano tesla).
In the above-described description, T represents Tesla which is the unit of the strength of a magnetic field, and 1 T=10000 Gauss and 1 mT=10 Gauss.
For allowing the change rate of a maximum magnetic field as the entirety of the display apparatus to satisfy the value lower than 25 mT/sec, it is indicated by the display manufacturers and sellers (e.g., IBM, etc.) that the change rate of the maximum magnetic field should be a value lower than 15 mT/sec as the standard requirement in the state of the cathode ray tube to which the deflection yoke is attached, i.e., the cathode ray tube display apparatus what is called an ITC (Integrated Tube Component).
For conventional technologies for suppression of leakage magnetic flux from the deflection yoke on the display surface of the cathode ray tube, there were two prior arts described below.
The first prior art is the technology shown in FIGS. 1A to 1C. This has been already filed at the Japanese Patent Office by N. V. Philips Gloellampenfabrieken (Japanese Patent Application Laid Open No. 223952/1987).
Namely, FIGS. 1A to 1C show examples where an electric wire 1 is electrically coupled to the deflection unit and the electric wire 1 is disposed on a face-plate 2, respectively. In these figures, reference numerals 3a and 3b represent ordinary connection terminals of the deflection unit, respectively.
In the case of the example of FIG. 4A (system I), the electric wire 1 is connected in series with deflection coils 4a and 4b, and two electric wire sections 1a and 1b in a horizontal direction of the electric wire are directly attached to the upper and lower side edge portions of the face-plate 2, or are attached in a manner that they are extremely close thereto, respectively.
In the case of the example of FIG. 1C (system II), for compensating a stray magnetic flux of individual deflection coils, a deflection coil 4a is coupled in series with the upper side electric wire section 1a in a horizontal direction, and a deflection coil 4b is coupled in series with the lower side electric wire section 1b in the horizontal direction.
In the case of the example of FIG. 1B (system III), a control current source 5 is disposed between the deflection coils 4a and 4b and the electric wire sections 1a and 1b. In this case, the electric wire sections are wound a plurality of times, i.e., as a plurality of loops at the upper and lower side edge portions of the faceplate 2.
The second technology has been proposed by the same assignee as one of this invention. (Japanese Patent Application No. 156411/1987, EPC Application No. 89306338.8).
Namely, the example shown in FIG. 2 (system IV) is as follows. In this figure, reference numeral 6 represents a cathode ray tube, reference numeral 7 a deflection coil, and reference numerals 8a and 8b electric wires for producing cancellation magnetic flux, respectively. The electric wires 8a and 8b are disposed in a manner that they are wound on the upper and lower halves around the display surface 10, respectively. A horizontal deflection current is delivered in a direction indicated by an arrow in the figure. The deflection coil 7 is provided in the vicinity of an electron gun 9 so that is deflects an electron beam. A horizontal deflection current is delivered from the deflection circuit to the deflection coil 7.
At this time, a signal from the deflection circuit is delivered to the deflection coil 7, and this signal is also delivered to the electric wires 8a and 8b, and varies in synchronism with the signal flowing in the deflection coil 7.
The leakage magnetic flux was measured under the following condition in connection with the systems I to IV according to the above-described first and second prior arts.
For measuring the recommendated value announced by the above SSI in Sweden, a meter (MAGNETIC FIELD METER MODEL MFM 1000) capable of measuring a low frequency leakage magnetic field developed by Combinova Company in Sweden is used. An actual measurement was conducted as follows. As shown in FIG. 3, antennas corresponding to magnetic fields from three directions were installed at positions of A, C and E. The CRT display apparatus was then rotated clockwise and anticlockwise using the central point of the display apparatus as a center. The change rate of the magnetic field and the strength of the magnetic field under the condition in FIGS. 3 and 4 (particularly at positions and angles of the antenna relative to the cathode ray tube), i.e., at the positions where antennas are installed, labelled A0, A1, A2, A14, A15, C0, C1, C2, C14, C15, E0, E1, E2, E14, E15, are measured by allowing a sawtooth current 7.8 A p-p having a horizontal deflection frequency of 20 KHz to flow in a horizontal deflection coil (not shown) of the deflection yoke 12 of the cathode ray tube 11, and by allowing a sawtooth current having a vertical deflection frequency of 60 Hz to flow in a vertical deflection coil (not shown).
As a result, the maximum values and the minimum values of measured values at 15 positions in accordance with the above-described systems I to IV are as shown in FIG. 5.
All the measured values shown in this figure vary to much extent depending upon the wiring position of a lead wire for conducting a current into the electric wire for cancellation of leakage magnetic flux provided in the vicinity of the face-plate of the cathode ray tube. By changing the wiring position of the lead wire, a value at a specific position on the above-mentioned electric wire may be minimized. However, if measurement is made with the lead wire in the above-described first and second prior arts hanging down in a direction of gravity, it was impossible to minimize, in a well-balanced manner, measured values on the measurement spherical body constituted by the above-described 15 positions. Further, cathode ray tube display apparatus could be hardly mass-produced in such an unbalanced state.
Meanwhile, when, as in the above-described arrangement shown in FIGS. 1A and 1B, two electric wire sections 1a and 1b for cancellation of leakage magnetic flux along the upper and lower side edge portions of the face-plate 2 are connected by electric wires indicated by broken lines, which are subjected to wiring on the diagonal line, a magnetic flux in the same direction as that of a magnetic flux (leakage flux) produced form the deflection coils 4a, 4b is produced across the electric wire on the diagonal line. Not only the leakage magnetic flux cancellation effect by the electric wire sections 1a and 1b can be reduced, but also it could not be expected to uniformly exhibit the leakage magnetic flux cancellation effect on the above-described spherical body designated by the SSI institution.
Further, also in the arrangement shown in FIG. 1C, a magnetic flux generated form the electric wire indicated by broken lines becomes effective in a direction of decreasing the leakage magnetic flux cancellation effect. Thus, a sufficient effect could not be expected.
Furthermore, in the above-described second prior art (EPC Appln. No. 89306338.8), it is suggested that the source from which a leakage magnetic flux is produced in front of the display surface of the cathode ray tube is a bend-up portion of the horizontal deflection coil.
However, as shown in FIG. 8 which will be described later, how an actual leakage magnetic flux is produced is such that leakage magnetic flux B.sub.2b radiated in a forward direction of the fluorescent display surface 53a is extremely greater than the magnetic flux B.sub.1 produced form the bend-up portions 50.sub.a1 and 50.sub.b1 of the horizontal deflection coils 50a and 50b. Thus, it is necessary to produce a cancellation magnetic flux in a direction opposite to that of the second prior art.
When a leakage magnetic flux cancellation magnetic flux is produced in a direction as indicated by the patent specification according to the above-described second prior art, a far greater leakage magnetic flux would be produced as compared to the case where there is no electric wire for cancellation of leakage magnetic flux.
In addition, as shown in FIGS. 11, 12 and 17 which will be described later, a leakage magnetic flux B.sub.2c in the same direction as that of the main magnetic flux B.sub.2a component is produced also from the side surface portions of the cathode ray tube or the side surface portions of the deflection yoke. Since this leakage magnetic flux B.sub.2c exerts an effect also in the forward direction of the display surface, unless such a leakage magnetic flux B.sub.2c is canceled along with other leakage magnetic flux, the leakage magnetic flux of the entirety of the cathode ray tube display apparatus cannot be reduced to a low level. Particularly cancellation of the leakage magnetic flux B.sub.2c toward the side surfaces could not be imagined by the prior art.
Accordingly, none of the above-described prior arts can reduce the strength of an unnecessary magnetic flux over the entire periphery of the cathode ray tube, and manufacture apparatus which can satisfy the standard requirement of recent display manufacturers and sellers. Thus, it was necessary to stably reduce the leakage magnetic flux radiated from the cathode ray tube display apparatus and to allow the value of the leakage magnetic flux to fall within the above-mentioned recommended value.